The development of efficient and effective insulating materials has been the subject of substantial commercial interest. This is particularly true for materials which are thermal insulators, e.g. which reduce the rate of heat loss (or gain) of any device, construct, or container. Foams are broadly used as insulators. References describing such uses and properties of foams include Oertel, G. "Polyurethane Handbook" Hanser Publishers, Munich, 1985, and Gibson, L. J.; Ashby, M. F. "Cellular Solids. Structure and Properties" Pergamon Press, Oxford, 1988. The term "insulator" refers to any material which reduces the transfer of energy from one location to another. Such energy may be include heat, acoustic, and/or mechanical types. Heat insulation is of particular importance and relates to the thermal conductivity of the insulating medium.
The "perfect" insulator is a vacuum. Developing and maintaining an evacuated space around the area to be insulated can, however, be impractical, particularly for larger volumes. The structural integrity required to withstand atmospheric pressure acting upon a vacuum vessel can be an issue.
A common insulating medium is a foam or cellular material having porous regions surrounded by a solid that provides integrity. The function of the foam as an insulator is to trap air and reduce thermal conductivity of the types described above. Foams are generally characterized by the size of the pores or cells within the structure as well as their density, which approximates the ratio of open-to-solid structure within the foam.
The thermal conductivity of any foam depends on four characteristics:
1. convection through the pores; PA1 2. conduction through the gas; PA1 3. conduction through the polymer; and PA1 4. thermal radiation through the cell walls and across the cell voids. PA1 A) a specific surface area per foam volume of at least about 0.01 m.sup.2 /cc; PA1 B) a density of less than about 0.05 g/cc; and PA1 C) a glass transition temperature (Tg) of between about -20.degree. and 90.degree. C.
Convection via movement of a gas through pores of an insulating medium diminishes with cell sizes down to about 4 mm, below which it becomes negligible. Convection through pores is suppressed in cells smaller than 10 mm. Most foams have cells much smaller than mm scale dimensions.
Conduction through the gas typically can account for as much as two-thirds of the system's thermal conduction. For this reason, foams filled with low conductivity gases can be preferred, although the gas will typically exchange with the atmosphere over time. Conduction through the solid polymer is negligible with low density foams.
Thermal radiation can account for one-quarter to one-third of the thermal conductivity in a foam. (See Glicksman, L. R.; Torpey, M.; Marge, A. J. Cell. Plastics 1992, 28, 571 and DeVos, R.; Rosbotham, D.; Deschaght, J. ibid 1994, 30, 302.) Radiative heat transfer is highly dependent on the cell size of the foam and decreases with cell size (which preferably would be .ltoreq.100 .mu.m). Kodama et al. (ibid., 1995, 31, 24) report on improvements in the k factor (a measure of thermal insulation ability) of a series of polyurethane foams ("PUFs") as average cell sizes decreased from 350 .mu.m to 200 .mu.m at a density of 0.052 g/cc. Doerge reports that foams with densities lower than about 0.037 g/cc showed increases in thermal conductivity attributable to the increase in cell sizes that typically occurs at these lower densities (Doerge, H. P. ibid., 1992, 28, 115), in part due to the increasing transparency of the cell walls and cell wall rupture (allowing rapid diffusion of the low conductivity gas filler). The best insulating rigid foams are low density foams (ca. 0.03-0.07 g/cc for closed cell PUFs) having the smallest cells possible filled with a gas having a low coefficient of thermal conductivity (or no gas at all). Thus, it would be desirable to produce foams having both low density and very small cells, e.g. .ltoreq.100 .mu.m. Such foams apparently cannot be produced by state-of-the-art blown foam processes.
The historical approach to making insulating foams for the appliance industry (e.g. refrigerators, water heaters, etc.) has been to use chlorofluorocarbons (CFCs) as physical inflating agents, especially for foams based on polyurethane and polyisocyanate starting materials. The reported association between CFCs and ozone layer depletion has sharply curtailed their production and increased the need for alternate materials and/or methods for making foams. Alternate blowing agents such as carbon dioxide or pentane however develop less efficient insulating foams relative to those made with CFCs. This results from the difficulty in achieving the same fineness of microstructure and densities possible with CFC blown foams. See for examples Moore, S. E. J. Cell. Plastics 1994, 30, 494 and U.S. Pat. No. 5,034,424 (Wenning et al.) Jul. 23, 1991. See also Oertel, p 273; Gibson and Ashby, Chapter 7, p 201.
Polyurethane foams are perhaps the most broadly used type in such applications. The chemistry used in processing presents certain disadvantages including poor photostability (see Valentine, C.; Craig, T. A.; Hager, S. L. J. Cell. Plastics 1993, 29, 569), the inevitable existence of undesirable chemical residues in the foams (see U.S. Pat. No. 4,211,847 to Kehr et at., issued Jul. 8, 1980, and U.S. Pat. No. 4,439,553 to Guthrie et al., issued Mar. 27, 1984, describing efforts to minimize these residues), and the production of noxious gases developed during burning due to the presence of nitrogen atoms within the composition (see Hartzell, G. E. J. Cell. Plastics. 1992, 28, 330). This can be particularly problematic in accidents involving public conveyances such as boats, automobiles, trains or airplanes which may catch fire. Injuries and fatalities may result solely from inhalation of these noxious gases. See Gibson and Ashby Chapter 8, p 212. This can also be an issue when the foam is discarded into a waste stream that is to be incinerated.
The building insulation industry has widely used foamed polystyrene rigid panels (in addition to glass batting and blown cellulose insulation). Styrene foam panels are useful in that they are rigid and may be nailed during construction, are hydrophobic to provide moisture resistance (which otherwise diminishes insulation value), and are relatively inexpensive. See Oertel p 277. This material is also widely used in beverage cups and food containers. The cell sizes of these materials are typically in the 300-500 .mu.m range. Smaller celled polystyrene foams have been prepared using the Thermally Induced Phase Separation Process (TIPS) described in Chemtech 1991, 290 and U.S. Pat. No. 5,128,382 (Elliott, et al.) issued Jul. 7, 1992 incorporated herein by reference. An important issue in making polymeric insulating foams commercially attractive for use as insulators is economics. The economics of foams depend on the amount and cost of the monomers used, as well as the cost of converting the monomers to a usable polymeric foam. The effort to reduce the cost of such insulating foams, especially in terms of reducing the total amount of monomer used, can make it very difficult to achieve the desired insulation and mechanical properties.
Accordingly, it would be desirable to be able to make an open-celled insulating polymeric foam material that: (1) has adequate rigidity or flexibility according to the requirements of use; (2) can be made with relatively small cell sizes to limit thermal conductivity contributed by radiation; (3) can be made without chlorofluorocarbons or other gases which may induce undesired environmental problems; (4) contains no nitrogen chemically bound in the structure that, upon combustion, may release toxic gases; and (5) can be manufactured economically without sacrificing desired insulating and mechanical properties to an unacceptable degree.